在介绍接口Interface的主要功能之前，我们先介绍一下什么是紧耦合

class Program
    {
        static void Main(string[] args)
        {
            var engine = new Engine();
            var car = new Car(engine);
            car.Run(3);
            Console.WriteLine(car.Speed);
        }
    }

    class Engine
    {
        public int RPM { get; private set; }
        public void work(int gas)
        {
            this.RPM = 1000 * gas;
        }
    }

    class Car
    {
        private Engine _engine;
        public Car(Engine engine)
        {
            this._engine = engine;
        }
        public int Speed { get;private set; }

        public void Run(int gas)
        {
            _engine.work(gas);
            this.Speed = _engine.RPM / 100;
        }
    }

紧耦合的编程：
倘若Engine类出现错误，如误写为this.RPM = 0；
则在大项目中难以定位修改这个问题，因此引入接口减少耦合程度。

接口是一组契约，用来约束一组功能，这组功能的调用者是被约束的，被约束为只能调用这组接口里所规定的功能（接口里没包含的功能看不见也调用不了），而且调用者不用关心这些功能由谁提供，接口来保证这些功能都稳定可靠。

    class Program
    {
        static void Main(string[] args)
        {
            var user = new User(new Iphone());
            user.UsePhone();
        }
    }
    interface IPhone
    {
        void Dail();
        void PickUp();
        void Send();
        void Receive();
    }


    class User
    {
        private IPhone _phone;
        public User(IPhone phone)
        {
            this._phone = phone;
        }
        public void UsePhone()
        {
            _phone.Dail();
            _phone.Dail();
            _phone.Dail();
        }
    }

    class XiaoMiPhone:IPhone 
    {
        public void Dail() { Console.WriteLine("Are U Ok?"); }
        public void PickUp() { Console.WriteLine("Hello?"); }
        public void Send() { Console.WriteLine("Think You!"); }
        public void Receive() { Console.WriteLine("Think You Very Much!"); }
    }

    class Iphone : IPhone
    {
        public void Dail() { Console.WriteLine("Buy?"); }
        public void PickUp() { Console.WriteLine("Buy??"); }
        public void Send() { Console.WriteLine("Buy???"); }
        public void Receive() { Console.WriteLine("Buy????"); }
    }
}

通过接口只需要更改

var user = new User(new Iphone());
var user = new User(new XiaoMiPhone());

就可以换手机。
通过引入接口极大降低了耦合度，即为松耦合。